Method for producing improved an anisotropic magent through extrusion

ABSTRACT

A method for producing an improved anisotropic magnet through an extrusion process wherein the container involved in the extrusion process is first coated, on its internal surface, with a material to prevent the bonding of the interior surface to the particle charge to be extruded and, thereby, to prevent cracking due to tensile stresses. In addition, an improved anisotropic extrusion product, with enhanced magnetic properties, can be obtained by holding the temperature of the extrusion product at temperatures between 1000° F. and 1750° F. for between 1 and 10 hours.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under a smallbusiness research and development grant for “A Simple Process toManufacture Grain Aligned Permanent Magnets” awarded by the U.S.Department of Energy (Grant No. DE-FG02-97-ER82313). The Government hascertain rights to this invention.

FIELD OF THE INVENTION

[0002] This invention relates to a method for producing, by an extrusionprocess, an improved anisotropic magnet which is crack-free and hasenhanced magnetic properties.

BACKGROUND OF THE INVENTION

[0003] It is known to produce a fully dense permanent magnet alloyparticle having crystal alignment through an extrusion process whereby aparticle charge of a permanent magnet alloy composition is placed in acontainer and the container is evacuated, sealed and heated to anelevated temperature. The container is then extruded to achieve crystalalignment and to compact the charge to full density to produce thedesired fully dense body. U.S. Pat. No. 4,881,984 (Chandhok et al.)describes such a method.

[0004] In practice however, this method has suffered from disadvantageswhich have precluded use of the resulting magnet bodies. Morespecifically, the extruded magnet body has been prone to cracking due totensile stresses which are set up in the magnet body during cooling ofthe magnet after the extrusion process. During the extrusion process, amagnet body, which typically is comprised ofneodymium-iron-cobalt-boron-gallium (NdFeCoBGa) orprasedymium-iron-cobalt-copper-boron-gallium (PrFeCoCuBGa) alloys, bondswith the container involved in the extrusion process—typically acylindrical steel can. The coefficient of thermal expansion between thecontainer and the magnet charge are at variance such that theaforementioned tensile stresses are created. During cooling, thesestresses result in cracking of the finished product which, in turn, hasprevented the use of this process to be used commercially. Further,magnets produced through the extrusion processes of the prior art havelacked enhanced magnetic properties of similar magnets produced by othermeans.

SUMMARY OF THE INVENTION

[0005] It is the purpose of the present invention, therefore, to providean improved method for producing an anisotropic magnet through extrusionwhich reduces the tensile stresses that result in cracking of thefinished product. An additional object of this invention is to providean improved method for producing anisotropic magnet bodies wherein theinternal surface of the container involved in the extrusion process islined with a material to prevent the bonding of the container to themagnet charge and reduce resultant tensile stresses which are createdduring cooling. Several materials may be used for this coating,including inert materials such as fused silica, high temperaturerefractory cement and boron nitride. These materials may be applied as apaste or spray. In a preferred embodiment, boron nitride provides asmooth surface on the final magnetic body. Other possible coatingmaterials include zirconia, yttria, ceria and other rare earth oxidesand their combinations.

[0006] The improved extrusion process described by this invention hasdirect application to the production of NdFeB anisotropic magnets.Additionally, the improved extrusion process described by this inventionmay be used to fabricate hexagonal ferrite magnets (i.e., barium,strontium and lanthanum ferrites) and provide high energy products inthe range 2-5 MG Oe and, also, samarium—iron nitrides of the compositionSm₂Fe₁₇N_(x) and MnBi magnets.

[0007] It is the further object of this invention to provide an improvedmethod for producing an anisotropic magnet body through an extrusionprocess wherein the extrusion product is subjected to a post-extrusionthermal treatment to improve and enhance the permanent magnet propertiessuch as remanence, coercivity and energy product. In particular, thepost-extrusion thermal treatment consists of holding the extrusionproduct at temperatures from 1000° F. to 1750° F. for 1 to 10 hours.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] It is known in the prior art to produce anisotropic magnetsthrough an extrusion process whereby a particle charge of the desiredpermanent magnet alloy is placed in a container, the container is sealedand heated (along with the charge) and extruded to achieve a magnethaving crystal alignment and full density. An example of the prior artin this regard is Chandhok et al. Unfortunately, and as discussed above,the finished product resulting from this methodology has yet to be usedcommercially because of tensile stresses and cracking which result inthis extruded magnet body. Such tensile stresses and related crackingoccur because of variances between the coefficient of thermal expansionbetween the container involved in the extrusion process and the magneticalloy. More specifically, the charge and resulting magnetic body bondwith the container during the extrusion process and, upon cooling afterthe extrusion process, temperature tensile stresses are set up in themagnetic body which result in cracks.

[0009] The above-described tensile stresses, and resulting cracks in thefinished magnetic body, can be prevented through use of a coating on theinternal surface of the container involved in the extrusion process.More specifically, in accordance with one embodiment of the invention,the container involved in the extrusion process can be coated with amaterial to prevent the bonding of the interior surface of the containerto the charge and resulting magnetic body. Preferably, the coating iscomprised of an inert material from the group consisting of fusedsilica, high temperature refractory cement and boron nitride. Thesematerials may be applied as a paste or spray. In a preferred embodimentof the present invention, boron nitride is used as the coating becauseit produces a final magnetic body which has a smoother surface.Additionally, the coating of the internal surface of the containerinvolved in the extrusion process can also be comprised of the groupconsisting of zirconia, yttria, ceria and other rare earth oxides andtheir combinations.

[0010] This improved extrusion process has, in a preferred embodiment,application to Nd(Fe,Co) (B,Ga) magnets. However, this improvedextrusion process is also applicable to fabrication of hexagonalferrites (i.e., barium, strontium and lanthanum ferrites) and, inparticular, hexagonal ferrites that produce high energy products in therange of 2-5 MG Oe and Sm₂Fe₁₇N_(x) and MnBi magnets.

[0011] Additionally, the magnetic properties of an extruded anisotropicmagnet can be improved or enhanced through a post-extrusion thermaltreatment. More specifically, in accordance with another embodiment ofthe present invention, the extrusion product, i.e. the resultingmagnetic body, can be subjected to a post-extrusion thermal treatmentwhich consists of holding the extrusion product at temperatures frombetween 1000° F. to 1750° F. for several hours. Through thispost-extrusion treatment, enhanced magnetic properties such asremanence, coercivity and energy product are achieved.

What is claimed is:
 1. A method for producing an improved anisotropicmagnet, said method comprising placing a particle charge of acomposition from which said magnet is to be produced in a container,evacuating and sealing said container, heating said container and saidparticle charge and extruding said container and particle charge,wherein said container, prior to introduction of said particle charge,is coated on the internal surface of said container with a material toprevent the bonding of said interior surface of said container to saidparticle charge and, thereby, to prevent cracking due to tensilestresses.
 2. The method of claim 1 wherein said material is an inertmaterial selected from the group consisting of fused silica, hightemperature refractory cement and boron nitride.
 3. The method of claim1 wherein said material is an inert material selected from the groupconsisting of zirconia, yttria, ceria, other rare earth oxides andcombinations of zirconia, yttria, ceria and other rare earth oxides. 4.The method of claim 1 wherein said coating is applied as a paste.
 5. Themethod of claim 1 wherein said coating is applied as a spray.
 6. Themethod of claim 1 wherein said method further includes the step ofsubjecting the post extrusion product to a thermal treatment whichconsists of holding the temperature of said extrusion product attemperatures between 1000° F. and 1750° F. to obtain enhanced magneticproperties.
 7. The method of claim 6 wherein said thermal treatment isapplied for between 1 and 10 hours.
 8. A method for producing animproved anisotropic magnet with enhanced magnetic properties, saidmethod comprising placing a particle charge of a composition from whichsaid magnet is to be produced in a container, evacuating and sealingsaid container, heating said container and said particle charge,extruding said container and particle charge, and subjecting the postextrusion product to a thermal treatment which consists of holding thetemperature of said extrusion product at temperatures between 1000° F.and 1750° F.
 9. The method of claim 8 wherein said thermal treatment isapplied for between 1 and 10 hours.